Etching methods for controlled cleaning and polishing of the surface of magnesium



nited States atent ETCHING METHOD FQR C-QNTROLLED CLEAN- ING AND POLISHING OF THE SURFAQE @F MAGNESIUM Adolf Hermann Wislren, Hamburg, Germany, assignor to Magna Kleen Inc., New York, N. kl, a corporation of New York No Drawing. Application May 4, 1956 Serial No. 582,663

11 Claims. (Cl. 41-42) The present invention relates to etching methods for controlled cleaning and polishing of the surface of magnesium and its alloys in the form of magnesium sheets and prefabricated parts.

Because of the extremely reactive nature of magnesium metal and its alloys which causes it to corrode rapidly and extensively, it has not been possible to exploit to the fullest capabilities its desirable lightness (about 35% lighter than aluminum), high tensile strength (about twice that of aluminum), its excellent surface hardness and the readiness with which it is formed in sheet or complex fabricated parts. These forms of magnesium are extensively employed in aircraft construction and in the manufacture of internal combustion engines; particularly, the alloys of magnesium with from 4 to of aluminum and which may contain varying small amounts (about ,2%10%) of one or more of copper, cadmium, zinc, manganese, silicon, etc. are of great value where lightness combined with strength are of importance. Since the attainment of clean, etched, polished surfaces protected against corrosion and oxidation and suitable for welding operations is of technical importance to the commercial development of the various forms of magnesium and its alloys, the development of commercial etching processes for simply, efficiently and rapidly obtaining the desired surface is a practical need which is satisfied by the present invention.

Heretofore etching has been carried out by using a strong oxidizing mineral acid such as dilute nitric acid or as a complete alternative an acid which attacks magnesium vigorously such as dilute hydrofluoric acid. It has been recognized that etching acids suitable for use with aluminum are not suitable with magnesium, because they act so vigorously in the latter to produce deep pits or blind holes containing corrosive acids which continue to eat away the metal, and hence require careful neutralizing, precise adjustment of the acid concentration, time, temperature and the other variables known in the art to produce an acceptable product.

Attempts have been made to tailor for specific magnesium alloys rather complex mixtures of acids containing nitric acid, hydrochloric'acid, sulphuric acid, soap, po tassiurn dichromate, etc., but these have very limited application, are difficult'to formulate, difiicult to control, and may product variable results when unskilled operators are employed or when a different magnesium alloy is usedv These ditliculties have been recognized in the prior'art which has sought to obviate them by employing soaps or high molecular weight fatty acids to restrain or slow down the vigor of etching, but the desiderata of quick, economical and uniform control of etching'using various kinds of magnesium'alloy has not been satisfactorily attained inthe aqueous systems employed heretofore.

It is surprising that the etching compositions of the present invention and the simple dipping non-electrolytic method employed, which combine vigorous etching acids in a substantially non-aqueous system should produce 2,849,297 l atented Aug. 26, 1958 ice ' positive assurance that a desirable clean, inert uniform oxide surface is left on the surface of the magnesium which further protects said surface against rapid oxidation.

An object of the invention is to provide substantially non-aqueous etching and polishing composition containing in parts by volume at least 65% of a water-miscible aliphatic alcohol solvent from about 10 to about 30% of an inorganic mineral acid etching agent containing two parts of nitric acid to one part of hydrochloric acid, while there may be present up to 5% of a water soluble lower aliphatic acid which assists etching, such as acetic acid.

A further object of the invention is to provide new and improved methods for the rapid, complete and efiicient etching of the surface of magnesium and its allows to provide corrosion resistant oxide coatings which are of uniform microscopic thickness and which are uniformly applied.

Other and further objects of the present invention will appear from the more detailed description set forth below, it being understood that such more detailed description is given by way of illustration and explanation only and not by way of limitation, since various changes therein may be made by those skilled in the art without departing from the scope and spirit of the present invention.

Alcohol ingredients:

By volume, percent Ethyl alcohol 35 Methyl alcohol 35 Ethylene glycol monoacetin ether 5 Mineral acid ingredients:

Nitric acid (specific gravity 20/4 C.=1.49-

1.50) 17.5 Hydrochloric acid (specific gravity at 20/ 40 The bath is maintained at a temperature of F.

Within one or two seconds, the surface of the mag nesium foil is agressively attacked by the mineral acid ingredient to remove metal, which goes into solution in the form of magnesium chloride and other salts of the mineral acid, and which liberates some bubbles of hydrogen, but it is noted that a few seconds after the initial agressive attack, and what appears to be almost simultaneously therewith the hydroxylic non-aqueous solvent appears to passivate the attack, reduces the aggressiveness of etching and bubble formation and appears to prevent further attack of the surface.

In effect, a stabilization or saturation point is quickly reached and the initial aggressive attack appears to be markedly slowed down so that a sample of the foil removed at this saturation stage reveals a continuous uniform although microscopically thin oxide film over the entire surface area of the foil exposed to the etching bath.

The foil removed from the bath from 2 to 10 seconds after immersion at a tempature of 100 F. is seen to exhibit this desired, polished uniform oxide coating, and it is completely and uniformly cleaned and polished in the manner above stated.

The foil after being removed from the etching bath is then rinsed, clean, with water, preferably with cold water or water at room temperature and is ready for whatever use is desired.

Remarkably, and despite the relatively high concentration of nitric and hydrochloric acid, the etching solution is not harmful to the skin if appropriate precautions are taken to prevent its remaining in contact with the skin for a prolonged period of time. In contrast to this many of the aqueous prior art etching baths cause severe burns when in contact with the skin for the same short time, even after they are rinsed from the skin in the same manner with cold water.

In the continuous operation described in the above example, the temperature of 100 F. is merely exemplary. The foil may likewise be etched at lower temperatures, such as room temperature (SS-70 F.) but the time is somewhat longer, about /2 minute to 2 minutes, the longer time with decrease in temperature being for the purpose of increasing slightly the microscopic thickness of the uniform oxide coating.

The amount of gases including hydrogen which are released in the etching bath is quite small and it is adequately drawn off without any hazard of explosion or fire by providing conventional blower or exhaust induced draft means over the bath to vent the gases to the ouside atmosphere.

Thus in the foregoing Example 1 the operation with strip or foil is continuous from the unrolling of the untreated unclean product through the degreasing, dewatering etching, rinsing and/or drying operations whereafter the polished microscopically oxide-coated foil is wound on windup rolls.

To facilitate economy, rapidity and efficiency of operation, it is desirable to arrange the magnesium article or articles being treated with respect to the preliminary cleaning (degreasing) apparatus and the etching bath at slightly elevated temperature in such manner that the temperature of the etching bath be kept at a precisely controlled uniform temperature, optimum for the oxide coating desired and the type of surface being etched. Accordingly, where a temperature of 100 F. as shown above is employed, the cleaning degreasing operation carried out at 85-90 F. will prevent overheating of the etching bath. Or if cleaning is carried out in an aqueous detergent near the boiling point of water, the parts or foil should be permitted to cool on the line to prevent overheating the etching bath.

The bath provides useful etching which is accelerated at increasing temperatures, up to 150 F. or more, depending upon the volatility of the alcoholic medium, while giving excellent control of etch depth and uniformity of coating. However, at temperatures above 150 F., the process may require modification by installing solvent recovery systems, bl'ower systems for removal of noxious vapors, etc., and the additional problems involvedat these temperatures above 150 F. do not provide any corresponding advantage in the type or kind of etching obtained. Accordingly, it is preferred to operate in a simpler and more flexible manner to adapt the process to the simplest kinds of equipment and thereby make it useful for small machine shops and metal fabricators.

EXAMPLE 2 Instead of using the etching bath A, the following bath is employed to clean, etch and polish cast magnesium parts of Dow metal containing 8% aluminum and 92% magnesium, which were first cleaned and degreased.

Etching bath B Alcoholic ingredients: By volume, percent 4 Mineral acid ingredient:

Nitric acid (sp. gr. 1.49) 11.0 Hydrochloric acid (sp. gr. 1.18) 4.8 Acetic acid 0.2

The temperature of the bath was 115 F. The parts were immersed for A2 to 2 minutes, washed clean and dried to provide an excellent coating of oxide as obtained and described in Example 1.

EXAMPLE 3 The same procedure as in Example 2 was carried out to clean, etch and polish drawn parts fabricated from an alloy containing 94.5% magnesium and 5.5% of cadmium and another set of parts made of an alloy containing 8.3% cadmium, 5% zinc and 0.2% manganese.

Both sets of parts were etched in an etching bath of the composition of etching bath A, Example 1, and in the composition of etching bath B, Example 2, for the same period of time, about 1 to 2 minutes at a temperature of 110 F. In both cases, excellent results were obtained, as in Example 1, but the results were achieved a little quicker with the acetic acid component as in etching bath B.

By adjusting the concentration of the acid ingredients and alcohol ingredients, further control is made possible and even the most troublesome cleaning and polishing operations may be carried out by such means as well as those control factors mentioned above.

For example, instead of using -98% HNO; in a proportion of 17.5 to 18% by volume of the total volume as shown above, as little as about 10% of nitric acid can be used, the remainder (78%) of this nitric acid component being water. The amount of water introduced thereby is 7.5% by volume.

The hydrochloric and ingredient in this example having a specific gravity of'l.18-1.19 contains per se about 65% of water so that the non-aqueou solution of this example contains about 4-5 of water by volume. Thus in the reduced acid concentration mentioned in the previous paragraph the ratio of nitric to hydrochloric acid being maintained at approximately 2 to 1 and the total acids being maintained at about from 1035% of the total of non-aqueous solvent and acid, it is seen that the amount of hydrochloric acid is reduced to 5% and the amount of water contributed by the hydrochloric acid drops from 45% down to 3%.

Thus the non-aqueous etching bath retains its desirable etching characteristics when the amount of water introduced with the mineral acid component is below 12%, preferably below 10% and when the minimum amount of mineral acids is about 10%made up of about 7%as nitric acid and about 3% as hydrochloric acid, this minimum acid component carries with it about 2-3% water.

Acetic acid may be added in an effective amount to improve etching of as little as 0.05% and up to 5%. Acetic acid being an acidic ingredient is grouped with the mineral acid constituents rather than with the nonaqueous organic ingredients. More than 5% may be present, but it is believed wasteful of the material and it is not necessary.

More than 30% total of the mineral acids, 21% of nitric acid and 9% of hydrochloric acid provides such an aggressive acid etching composition as to be undesirable in view of the fine control of the microscopic oxide coating which is to be attained, and which is also wasteful of the acid materials if the control of the coating is effected through the use of lower temperatures. It is thus seen that the maximum mineral acid composition containing 30% mineral acids and 5% of organic aliphatic acid such as acetic, monochloracetic, propionic or monochloropropionic acid would contain a minimum of 65% of the water miscible alcoholic components and a maximumm of 12% of water in such composition.

The foregoing examples illustrate water-miscible alcoholic substantially non-aqueous acid etching compositions in which about 5% of a water soluble glycol ether may be present as in etching bath A, Example 1, or in which said amount of ether is omitted as in etching bath B, example 2.

The water soluble glycol ether, ethylene glycol monoacetin ether is typical of the water soluble aliphatic polyhydroxy derivatives which in the amount employed, preferably 3-10% reduce the volatility of the alcoholic medium. Water-insoluble volatile ether such as diethyl ether are undesirable because of their volatility and because they interfere slightly with easy rinsing of the metal parts after etching. Other polyhydroxy ethers or polyhydroxy derivatives which may be employed are the carbitols, low molecular weight (300-400) polyethylene oxide ethers, cellosolve ethers and the like. The use of these ethers renders the etching process more satisfactory because of greater uniformity and repro ducability particularly at elevated temperatures of IOU-150 F.

The aliphatic monohydric alcohol components of the alcoholic medium may be varied in the mixture in accordance with the desired temperature of etching to control the volatility of the bath. At lower temperatures larger amounts of methyl alcohol may be used and at higher temperatures larger amounts of ethyl alcohol and isopropyl alcohol may be employed. Although straight ethyl or methyl alcohol may be used, these in admixture appear to provide for a longer bath life and at the same time reduce the toxic manifestations which are exhibited by methyl alcohol per se.

Having thus disclosed the invention what is claimed is:

1. A method of etching the surface-of degreased and cleaned fabricated parts or sheets of magnesium and its alloys comprising dipping said parts in a water-miscible alcoholic etching medium containing, in parts by volume, at least 65% of a water-miscible aliphatic alcohol about 30% of an inorganic mineral acid etching agent consisting of about 2 parts of nitric acid to 1 part of hydrochloric acid and up to 5% of a water-soluble lower aliphatic organic acid.

2. A method as claimed in claim 1 wherein said alcoholic etching medium comprises a mixture of different water-soluble monohydric aliphatic alcohols.

3. A method as claimed in claim 1 wherein said alcoholic etching medium consists essentially of about 35% of methyl alcohol, and about 35% of ethyl alcohol, about 5% of ethylene glycolmono acetin ether, about 12.5% of nitric acid and about 7.5% of hydrochloric acid.

4. A method as claimed in claim 1 wherein said alcoholic etching medium consists essentially of about of ethyl alcohol, 20% of isopropyl alcohol, 10% of ethylene glycolmono acetin ether, 11% nitric acid, about 4.8% hydrochloric acid and about 0.2% of acetic acid.

5. A method as claimed in claim 2 wherein said alcoholic etching medium contains about 310% of a water-miscible aliphatic polyhydroxy ether.

6. A method as claimed in claim 1 wherein said parts are clipped in said etching medium for a period of from two seconds to about 2 minutes at a temperature up to about 150 F. whereafter said parts are removed from the etching medium and rinsed with water to remove the etching medium therefrom.

7. A water-miscible alcoholic etching medium for etching the surface of degreased and cleaned fabricated parts or sheets of magnesium and its alloys by immersion into said medium consisting essentially, in parts by volume, of at least of a water-miscible aliphatic alcohol, about 10-30% of an inorganic mineral acid etching agent which consists of about 2 parts of nitric acid to 1 part of hydrochloric acid and up to 5% of a water-soluble lower aliphatic organic acid.

8. An etching medium as claimed in claim 7 wherein said alcoholic etching medium comprises a mixture of dilierent water-soluble monohydric aliphatic alcohols.

9. An etching medium as claimed in claim 7 wherein said alcoholic etching medium consists essentially of about 35% of methyl alcohol, about 35% of ethyl alcohol, about 5% of ethylene glycolmono acetin ether, about 12.5% of nitric acid and about 7.5% of hydrochloric acid.

10. An etching medium as claimed in claim 7 wherein said alcoholic etching medium consists essentially of about 55% of ethyl alcohol, 20% of isopropyl alcohol, 10% of ethylene glycolmono acetin ether, 11% nitric acid, about 4.8% of hydrochloric acid and about 0.2% of acetic acid.

11. An etching medium as claimed in claim 7 wherein said alcoholic etching medium contains about 3-10% of a water-miscible aliphatic polyhydroxy ether.

References Cited in the file of this patent UNITED STATES PATENTS 1,994,500 Boller Mar. 19, 1935 2,399,134 Miller Apr. 23, 1946 2,640,763 Easley et a1. June 2, 1953 FOREIGN PATENTS 500,009 Great Britain Feb. 1, 1939 

1. A METHOD OF ETCHING THE SURFACE OF DEGREASED AND CLEANED FABRICATED PARTS OR SHEETS OF MAGNESIUM AND ITS ALLOYS COMPRISING DIPPING SAID PARTS IN A WATER-MISCIBLE ALCOHOLIC ETCHING MEDIUM CONTAINING, IN PARTS BY VOLUME, AT LEAST 65% OF A WATER-MIXCIBLE ALIPHATIC ALCOHOL ABOUT 10-30% OF AN INORGANIC MINERAL ACID ETCHING AGENT CONSISTING OF ABOUT 2 PARTS OF NITRIC ACID TO 1 PART OF HYDROCHLORIC ACID AND UP TO 5% OF A WATER-SOLUBLE LOWR ALIPHATIC ORGANIC ACID. 